The invention relates to a spray powder for the manufacture of a thermally insulating layer which remains resistant to high temperatures. It relates to a method for the manufacture of the spray powder in accordance with the invention and also to a substrate coated by means of a thermal spraying process and using the spray powder in accordance with the invention. The substrate is a substance from which, for example, the blade of a gas turbine wheel is made.
A thermally insulating layer of this kind is termed TBC (“thermal barrier coating”. The substrate onto which the TBC is sprayed can already be coated with a single- or multi-layered partial coating, in particular a primer. A least one thermally insulating functional material is used as a coating material, which on the one hand has a strikingly lower thermal conductivity than the substrate and, on the other hand, forms a chemically and thermally stable phase at high temperatures.
Characteristics of a coating of the type TBC, its possible material composition and also problems relating to the ageing of this coating are known from EP-A-1 225 251. In this publication the main emphasis is on coatings with columnar microstructures, which can be manufactured by means of processes in which the functional material—advantageously YSZ (zirconium oxide, which is stabilized with yttrium)—is vaporized and condensed out on the surface to be coated. Such processes are PVD or sputter processes for example. Non-columnar coatings, which are likewise discussed in EP-A-1 225 251, result during thermal spraying processes using suitable powder mixtures. During thermal spraying processes an anisotropic, inhomogeneous microstructure is formed with granules, at the boundaries of which micro-pores occur, in particular also gap-shaped micro-pores.
EP-A-1 225 251 mentions the ageing of the coatings: The relatively low thermal insulation of the TBC is concerned with inhomogeneities of the microstructure, which is given by a plurality of crystal granules, wherein the boundary zones between the granules are decisive. The local density is less in these boundary zones than inside the crystals. The micro-pores and lattice defects inside the granules also have a lowering effect on the thermal conductivity. As regards the ageing processes, these are thickenings of the microstructure, which result at high temperatures due to a sintering together, namely a homogenizing growing together of micro-pores at the granule boundaries. The thermal conductivity, which should remain as low as possible, increases with higher compression. Contaminants which are present due to silicon, titanium, iron, nickel, sodium, lithium, copper, manganese, potassium and/or oxides of some of these elements result in amorphous phases, which form thin films at the granule boundaries. Amorphous phases of this kind encourage the homogenization of the coating on the basis of a sintering together of the granules. The homogenization processes can be eliminated, prevented or at least slowed down with suitable additives. An additive of this kind is aluminum oxide, which is present in the form of precipitated crystallites. These can bind the named contaminants and in addition fix the micro-pores which are located between the granules. The aluminum oxide absorbs silicates out of the films, which bind the neighboring granules. Thus gap-like empty cavities form between the neighboring granules which represent barriers for a transport of heat.